The inventions herein pertain to the field of communication networks, including methods and apparatus for acquiring and maintaining synchronization in a wireless communication network.
In a typical wireless communication network, one or more base stations are selectively positioned within respective, defined geographic areas or cells, and are used to transmit and receive communication signals to and from, respectively, one or more remote stations, (e.g., mobile or cellular telephone handsets), located within the respective cell. In particular, the base stations act as both intermediary points by which a communication path may be periodically established and maintained with respective remote stations, as well as end points of a hierarchical stationary network, which also includes an overlay or backbone network, such as, e.g., a public switched telephone network (xe2x80x9cPSTNxe2x80x9d).
A selected communication protocol defines a method in which the various remote stations can communicate with one or more base stations of the communication network, e.g., in order to place and receive telephone calls. The communication protocol will preferably provide air-channel agility between respective base stations and remote stations, while also providing a secure voice or data link. A fundamental concern of the selected communication protocol for a network is the ability of the remote stations to communicate with the base stations in a simple, flexible and rapid manner, e.g., so that a remote station is not required to wait to establish a communication path, and/or so that a hand-off of an active call between base stations in a mobile network is transparent to a respective remote station. In this respect, the ability to acquire and maintain synchronization between a base station and a mobile station is an important consideration. Further, network-wide synchronization should be established and maintained for optimal operation of a mobile communication network, e.g., to minimize interference problems otherwise caused by non-synchronized base and/or mobile station transmissions in the same, or adjacent, cell location(s).
For example, in U.S. patent application Ser. No. 08/284,053, assigned to the assignee of the present application and which is hereby fully incorporated herein by reference, a protocol for a wireless communication network is described for use with a wireless communication network, wherein each base station transmits over a set of time-division air channels, or time slots, by transmitting in time slots in sequence, referred to herein as an over the air loop. Each base station time slot polling transmission is followed by a first gap (or xe2x80x9cguard timexe2x80x9d), a remote station transmission (if a remote station attempts to communicate), and a second (guard time) gap, before the base station transmits over the next time slot. A remote station receiving a base station polling transmission in an (unoccupied) time slot, may then transmit information to the base station over that respective channel, e.g., to establish a communication link with that base station via the respective polling path. Each base station may thereby maintain communication with as many remote stations as there are available time slots in its over the air loop.
In accordance with this protocol, handoffs between base stations are preferably initiated by the respective mobile station, which monitors available time slots from the same and competing base stations during unused time slots. A mobile station may handoff within the same over the air loop to establish communication in a new time slot of the same base station, or may handoff in such a manner to establish communication within a over the air loop of a different base station. In the latter case, a xe2x80x9cbase station controllerxe2x80x9d may assist in transferring the call from one base station to another.
Successful operation of the aforedescribed protocol depends on the stability of respective internal base station and mobile station transmission timing. In particular, in order for respective base station and mobile station transmissions to stay locked to each other within the respective time slot interval (s), the mobile station transmission timing must be synchronized with the base station transmission timing. For example, in U.S. Pat. No. 4,494,211, issued to Schwartz, a synchronization system for a xe2x80x9cmaster/slavexe2x80x9d satellite pair is described whereby each satellite of the pair transmits timing signals synchronized to its own respective clock, receives the timing signals from the other satellite and independently measures the difference between the transmission and reception of the respective timing signals. The time difference measured by each satellite is then transmitted to the other, wherein each utilizes the respective time difference measurements to calculate the asynchronism between the respective clocks and the range between the satellites. Based on those calculations, the xe2x80x9cslavexe2x80x9d satellite clock is then adjusted so that the calculated asynchronism is reduced to within an acceptable difference. As can be seen, the Schwartz synchronization system is both complex and requires periodic signaling bandwidth in both directions dedicated to the transmission of clock signals and respective measurement calculations.
Thus, it would be advantageous to provide a method and network architecture for the simple and rapid acquisition and maintenance of synchronization between respective base stations and mobile stations of a mobile communication network, without adversely impacting over-the-air bandwidth or the robustness of an established communication path. It would be further advantageous to provide the capability in a mobile communication network for network-wide synchronization, including both xe2x80x9cintra-networkxe2x80x9d synchronization, i.e., between respective elements of the mobile network itself, and xe2x80x9cinter-networkxe2x80x9d synchronization, i.e., between the mobile network and a respective linking overlay network.
The present invention is directed toward a method of synchronizing a base station and a remote station, wherein the base station is communicatively coupled with the remote station and a reference network. A base station clock signal is compared with a reference clock signal derived from the reference network and adjusted accordingly. The base station clock signal is then used to generate timing information in the form of a preamble, periodically transmitted over a wireless communication network to the remote station where a clock signal is generated. The remote station compares the clock signal with the timing information and adjusts the clock signal accordingly. This is done without reference to an external clock.
In one embodiment, a first preamble and a second preamble are generated at a base station and then the first preamble is transmitted from the base station. At a remote station, a clock signal is generated. After receiving the first preamble, yet prior to receiving the second clock signal, the remote station generates a clock control signal that is based on the clock signal and the first preamble. The clock signal is then adjusted accordingly, based on the clock control signal and without reference to an external clock.
Other and further aspects and features of the invention will become apparent from the following drawings and detailed description.